U.S. Pat. No. 6,359,276 discloses an infrared sensor unit composed of a thermal infrared sensor and a semiconductor device which are arranged in a side-by-side relation on top of a semiconductor substrate. The thermal infrared sensor is supported in the semiconductor substrate by means of porous thermal insulation support. The porous thermal insulation support is realized as a part of the semiconductor substrate for mounting thereon the infrared sensor in a thermally insulating relation from the remainder of the substrate. The porous thermal insulation support is composed of a sensor mount and a set of porous bridges connecting the sensor mount to the substrate. Each of the sensor mount and the bridges are formed by anodizing a top part of a doped region formed in the surface of the semiconductor substrate into a porous body. Thus, the prior art makes the best use of the semiconductor substrate carrying the semiconductor device to give the thermally insulation support for the infrared sensor. However, there remains a problem that the infrared sensor cannot be disposed immediately upwardly of the semiconductor device since the thermal insulation support is only made by anodizing the top surface of the semiconductor substrate at a portion horizontally spaced away from the semiconductor device. Due to this limitation, the infrared sensor unit is difficult to be made compact. Especially, when a plurality of the infrared sensor units are arranged in a two-dimensional array to constitute a thermal image sensor, the sensor units cannot be densely packed and therefore suffer from lowered resolution.